The present disclosure relates to automatic determination of printing traps.
Computer-graphics illustrations are typically made up of graphical elements of various types. Types of graphical elements include rasterized images, glyphs, vector strokes, vector fills, image masks, soft masks and gradients. A graphical element typically includes a path which defines the boundary of the graphical element. Graphical elements can interact with each other (e.g., by overlapping). Hereafter, any illustration discussed is a computer-graphics illustration unless otherwise noted.
When an illustration is printed, a printing device can print color in the illustration using multiple print units (e.g., individual printing plates in a multi-plate printing press). If the print units are not properly aligned, colorless gaps characteristically occur between adjacent portions of the illustration having different colors. Gaps can also arise if, during the printing process, the medium on which the illustration is being printed shifts, shrinks as ink dries, or expands as ink is applied. Gaps can also arise as a result of inaccuracies during intermediate stages of the printing process, such as stretching of a film negative used to produce a printing plate. To reduce the likelihood of gaps in a printed representation of an illustration, a process called trapping is commonly used. Trapping involves creating so-called traps prior to printing: overlaps of color or underlaps of color, sometimes called spreads and chokes. Spreads make a graphical element larger so that the edges of the object print over an underlying object; chokes make the obscured area of the underlying object smaller than the outline of the foreground object. A trapping process typically creates thin areas which overlap the color boundaries of pairs of adjacent graphical elements so that if the color of one graphical element is misaligned relative to the color of another graphical element during printing, the overlap will prevent the formation of a gap between the graphical elements where no color is printed.
The rules used to trap an illustration typically depend on many factors, including the colors and types of the graphical elements being trapped. For example, when two adjacent rasterized images are trapped, usually both rasterized images are adjusted to extend a small amount over a common boundary between the elements, resulting in a “centerline trap.” When two adjacent vector graphical elements (e.g., vector fills) are trapped, typically one graphical element is extended over the common boundary a larger amount, which results in a trap being located entirely on one side of the common boundary. Typically the graphical element of lighter color is spread into the graphical element of darker color. When a vector graphical element is trapped against a rasterized image, either a centerline trap can be used, so that the trap location does not change as the color of the image pixels change, or the lighter colored element can spread into the other element. Traps between gradient graphical elements and adjacent graphical elements can gradually move from one side of the boundary between graphical elements to the other as the color of the gradient changes, a so-called “sliding trap.”
Another trapping rule requires that lighter colors spread into darker colors. An exception is opaque spot colors, in which case any colors applied earlier in the printing process are spread under the opaque spot color. Another exception is rich black (black printed with a screen of another color to deepen the black), which is trapped by choking the screened color away from the edge of the rich black, leaving a border of plain black and thereby preventing the color from fringing along the black border. Typically, the color overlap resulting from trapping is not noticeable, however in situations such as when two abutting pastel spot colors are trapped, a dark line along the boundary can result from trapping. To mitigate the visual impact of such a dark line, the colors can be lightened along the edge.
The extent of trapping required for a given illustration depends on the medium (e.g., how much will the medium expand when wetted by ink), the resolution, calibration, and type of printing press, as well as such things as the order in which the medium will be run through the printing press units, as mentioned previously.
Trapping an illustration by hand is a time-consuming process. While a computer program can be used to trap an illustration automatically, automatic trapping typically is guided by trapping rules that depend on the types of graphical elements involved. In some situations, such as with computer illustrations containing drop shadows, previous automatic trapping techniques produce undesirable results. The areas around drop shadows are typically computed as raster images, which means that a vector object which passes near a drop shadow can show an abrupt change in the trapping, for example, from trapping on the side of the darker graphical element to centerline trapping at a certain distance from the drop shadow. The abrupt change in trapping can be especially noticeable on small vector text, which can be obliterated by centerline trapping.